1. Field of the Invention
The present invention relates to a connection between a first member and a bore formed in a second member which is such that said first and second members can be separated relatively easily for maintenance or repair. The connection of the present invention is especially useful for attaching a piston to a piston rod in hydraulic or pneumatic cylinders, and will be described with especial reference to this application, but in fact the present invention is also valuable in any application in which it is necessary to secure a first member in the bore of a second member, especially where very high axial thrusts are present.
2. Description of the Prior Art
Two of the most common methods of securing a piston to a piston rod are shown in FIGS. 1 and 2 of the accompanying drawings.
FIG. 1 shows a longitudinal section through a piston 2 which is secured to a piston rod 3 by a circlip 4. A groove 5 is formed around the circumference of the piston rod 3 with the inner edge of the groove abutting the edge of the piston. The circlip 4 is sprung into the groove 5 and the portion of the circlip which projects out of the groove holds the piston onto the rod. A corresponding circlip is mounted abutting the other end of the piston. In use, the circlip is subjected to forces which are predominantly shearing forces (arrows L) and the strength of the whole piston and piston rod assembly is limited by the strength of the circlip and/or the strength of the groove, in shear. This type of design is disclosed in U.S. Pat. Nos. 4,069,747 and 4,171,665.
FIG. 2 shows a side view of a piston rod 10 which has been machined to provide a reduced diameter portion 11 terminating in a screw-threaded portion 12. The piston (not shown) is mounted on the portion 11 and held in place by a locknut (not shown) screwed onto the portion 12, which forces the piston against shoulder 13.
This design has the following drawbacks:
(a) The strength of the piston rod is reduced by the formation of the reduced-diameter portion 11.
(b) It requires very accurate machining to form the portion 11 exactly concentric with the remainder of the piston rod, and if this is not achieved, the piston will be eccentric in the cylinder resulting in excessive seal and wear-ring wear, and also giving rise to shear forces on the shoulder 13.
(c) The locknut must be very tightly screwed onto the piston, and even so tends to shake loose in time, especially if subjected to frequent shock-loading. The prestress caused by torquing the locknut onto the piston rod reduces the overall strength of the piston and piston rod assembly.
It is also possible to secure the piston to the piston rods by welding (e.g. as disclosed in U.S. Pat. No. 3,312,150) but this has two major drawbacks:
1. To remove the piston from the rod, the weld must be cut away.
2. The heat of welding tends to distort the piston and the rod and/or to damage the material of which they are made.
In addition to the methods described above, a number of more elaborate methods have been proposed:
U.S. Pat. No. 2,853,347 discloses a pump piston which is screw-threaded to a piston rod. A rubber piston body member is secured to the piston head by circlip-type snap rings which lie in grooves formed on the piston head and abut against retainer plates on the body member. This design has the disadvantage that the screw-threaded connection between piston and rod tends to slacken in use, and the circlip connection between the piston body and head has the drawbacks already discussed.
U.S. Pat. No. 3,457,842 discloses a design in which the piston is retained on the piston rod by pairs of collets, one pair at each end of the piston. Part of each collet locates in a groove formed on the piston. This design has the drawback of the FIG. 1 design described above, in that the forces on the collets are predominently shear stresses, and has the additional drawback that, to secure the piston correctly, the outer surface of the retaining ring of each collet contacts the bore of the cylinder, thus increasing the bore wear rate.
U.S. Pat. No. 3,951,048 discloses a piston secured to a piston rod by circlips recessed into the faces of the piston. This arrangement has the drawbacks of the FIG. 1 design described above.
U.S. Pat. No. 4,317,409 discloses a piston and piston rod for a mud pump, in which the outer edge of each piston is tapered and designed to be expanded outwards towards the cylinder wall by mud pressure, to ensure a good seal between the piston and the cylinder wall. Each tapered portion is supported by a triangular cross-section pressure ring which is held in place by a circular cross section retainer ring which seats in a part-circular groove. The retainer ring does not contact the piston itself, but simply acts as a circlip with respect to the pressure ring. This design is useful only in applications where an extremely abrasive substance is being pumped and unusually heavy piston wear is anticipated, and is of no interest for more general applications.